A Molecular Perspective on Colistin and Klebsiella pneumoniae: Mode of Action, Resistance Genetics, and Phenotypic Susceptibility

Abstract

Klebsiella pneumoniae is a rod-shaped, encapsulated, Gram-negative bacteria associated with multiple nosocomial infections. Multidrug-resistant (MDR) K. pneumoniae strains have been increasing and the therapeutic options are increasingly limited. Colistin is a long-used, polycationic, heptapeptide that has regained attention due to its activity against Gram-negative bacteria, including the MDR K. pneumoniae strains. However, this antibiotic has a complex mode of action that is still under research along with numerous side-effects. The acquisition of colistin resistance is mainly associated with alteration of lipid A net charge through the addition of cationic groups synthesized by the gene products of a multi-genic regulatory network. Besides mutations in these chromosomal genes, colistin resistance can also be achieved through the acquisition of plasmid-encoded genes. Nevertheless, the diversity of molecular markers for colistin resistance along with some adverse colistin properties compromises the reliability of colistin-resistance monitorization methods. The present review is focused on the colistin action and molecular resistance mechanisms, along with specific limitations on drug susceptibility testing for K. pneumoniae.

Keywords: Klebsiella pneumoniae; colistin; lipid A; resistance mechanisms; susceptibility testing.

Figure 1

Colistin chemical structure divided in three portions: hydrophobic acyl tail segment of 6-methyloctanoic acid (A); linear tripeptide tail segment of diaminobutyric acid and one threonine residues (B); hydrophobic heptapeptide ring of diaminobutyric acid, leucine and threonine residues (C). Positively charged amine groups are highlighted in red [13,17,18].

Figure 2

Colistin mechanisms of action: (A) lysis mechanism; (B) vesicle-vesicle contact pathway; (C) Inner membrane lipid A targeting. In OM, colistin induces the displacement of calcium and magnesium cations from lipid A in order to bind it (1). This electrostatic interaction, with the help of the hydrophobic regions of colistin, weakens the OM structure providing colistin access to periplasm (2). There, the way how colistin breaks IM is explained by the three hypotheses: the lysis mechanism (A), where colistin straddles the phospholipid bilayer decreasing the IM thickness and leading to cell lyses (3) [21,25]; a vesicle-vesicle contact pathway (B), where the colistin acyl tail induces the exchange of phospholipids between the outer leaflet of IM and inner leaflet of OM, resulting in the structure instability of theses membrane and cell death (4) [18,21,24]; fand, through inner membrane lipid A targeting (C), where colistin targets the lipid A molecules that are transiently in IM, after being translocated by MsbA from the cytoplasm (where they are synthesized) and before being transported to OM, which will induce cytoplasmic content leakage and consequently cell death (5) [26,27]. LPS—lipopolysaccharide; OM—outer membrane; IM—inner membrane.

Figure 3

Regulatory network of Lipid A modification in Klebsiella pneumoniae.

Figure 4

Gene regulatory and plasmid-encoding pathways of LPS modification in Klebsiella pneumoniae.

Figure 5

Maximum-likelihood phylogenetic tree based for the Mcr amino acid sequences (Table S1) and aligned using Seaview 4.0 [87]. The maximum-likelihood tree was obtained using PhyML as implemented in Seaview using the BLOSUM62 substitution matrix allowing for across site rate variation. The tree was annotated using the Interactive Tree of Life online tool [88].